Rabu, 31 Agustus 2011

On old-fashioned dynamometers, the operator or the observer had to manually record the simultaneous readings on paper.
Nowadays, most dynamometers replace the observer’s notes with computerized devices that acquire and store the data.
Because of the very loud noise generated by the dynamometer and the engine, a performance test is stressful to anyone who’s watching it. In these given conditions, it is very difficult to manually record all the data.
A reliable computerized data acquisition system is considered to be indispensable today among engine builders and testers. Nowadays it is possible to buy recording, control, and playback capabilities in a palm sized package, that many years ago would have cost a lot more and would have had the size of a refrigerator.
An appropriate computerized data acquisition system must have a very fast sampling rate. For obtaining quality data, you will need at least 100 samples, of all sensor channels, per second (100Hz). A 200Hz logging rate or more is even better. It is important to remember that between spark plug firings there is a drop in torque and rpm that can be measured. The crankshaft is accelerated right after combustion occurs and afterwards it slows down until the plug will fire one more time. It is not possible to detect these rapid highs and lows when you are driving on the track or on the road, but the dynamometer will.
Therefore, a 200 Hz logging rate for your data acquisition system should do the trick, as well as an appropriate dynamometer.
However, if you sample at only 50Hz, you should consider that you are recording a single torque and rpm sample every other revolution. From time to time, a series of samples will synchronize with the firings of the plugs, and sometimes the data samples will fall in synch with some lower power compression strokes. If you are using a fast and reliable acquisition system that read each cycle multiple times, then the data captured can be used to average out the given phenomenon.
Averaging the data is a very important step in data acquisition.
At this point, while experienced dynamometer operators see an identical power curve in both graphs, inexperienced dyno operators expect to see a smooth, clear line.
The reason why the ability to average and dampen the data is essential for the acquisition system is that you don’t have to deal with so many printings.
For instance, at 100Hz you're getting 1000 lines of data for even a ten-second dynamometer pull. You might choose not to search for any small changes in dozens of pages of data every time you want to prepare another run. Averaging both eliminates transient noise and produces clear printouts, and thus it is very useful in the process of logging the data from the dynamometer.